1. Field of the Invention
The present invention relates to an image forming apparatus which is utilized as a copying machine, a laser printer, a facsimile machine, or a similar apparatus, and more particularly, to an image forming apparatus preferably utilized as a Tandem-type color copying machine.
2. Description of the Related Art
FIG. 13 shows a schematic cross-sectional side view of a printer unit of a Tandem-type color copying machine. As shown in FIG. 13, the copying machine is provided with an endless transferring belt 10 and photosensitive drums 2Y, 2M, 2C, 2K. The photosensitive drums 2Y, 2M, 2C, 2K are disposed along the traveling path of the transferring belt 10 and correspond to the colors, yellow (Y), magenta (M), cyan (C), and black (K), respectively. A paper-feed unit 5 and resist rollers 6 are disposed at the upstream portion of the transferring belt 10. Further, fixing rollers 7 are provided at the down-stream portion of the transferring belt 10.
In this copying machine, the scanner(not shown) reads the original image, and the image data are subjected to a predetermined process to form toner images on the photosensitive drums 2Y, 2M, 2C and 2K corresponding to the colors Y, M, C and K, respectively. The transfer sheet 11 as a recording medium is fed onto the transferring belt 10 through the resist rollers 6 from the paper-feed unit 5, and is electrostatically adhered to the transferring belt 10, whereby the transfer sheet 11 is transported by the transferring belt 10. The respective color toner image formed on each photosensitive drum 2Y, 2M, 2C, 2K is transferred to the transfer sheet 11 one on another, and then the transferred images are fixed by the fixing rollers 7, whereby a full-color image is obtained.
Though this Tandem-type color copying machine has high-speed processing ability compared to another types of copying machines, it is expected that the Tandem-type color copying machine with higher-speed processing ability is developed.
The improved processing speed may be obtained by improving the copy processing speed, (e.g., transferring speed and the image processing speed), or by shortening the paper-feed interval.
However, in the former method which improves the copy processing speed, there will be a disadvantage that the load on electronically photographing becomes large. Therefore, relatively speaking, it is preferable to employ the latter method which shortens the paper-feed interval to improve the processing efficiency. However, it is difficult to shorten the paper-feed interval because of the reasons detailed below.
The transferring belt 10 of the Tandem-type copying machine is usually formed by overlapping and joining both longitudinal ends of a dielectric resin sheet made of, for example, polyethylene terephthalate resin or polyvinylidene fluoride resin to form an endless belt. Thus, the transferring belt 10 has a seam 10a.
The characteristic of the seam 10a is different from that of the other portion. Accordingly, if the transfer sheet 11 is fed onto the transferring belt 10 such that the transfer sheet 11 overlaps the seam, the image transferring sometimes cannot be preferably performed to cause an inappropriate image transfer, resulting in poor images on the transfer sheet 10.
Alternatively, an endless transferring belt may be formed by using molding dies. However, weld lines of the molding dies may appear on the belt. The characteristic of the weld lines is different from that of the remaining portion of the belt. The specific portion such as a seam or weld lines is hereinafter referred to as a seam.
Though it is possible to form an endless transferring belt with no seam, the cost for manufacturing the molding dies will be expensive. Further, in order to form a long endless transferring belt, large molding dies which are difficult to manufacture are required. Furthermore, such molding dies include a plurality of parts, which causes uneven thickness of the belt. Such uneven thickness affects the resist accuracy which is the most important factor in the full-color copying machine. Accordingly, under the present circumstances, an endless transferring belt is only used as a short length transferring belt in a small-size copying machine.
Under such circumstances, in a normal copying machine, it is controlled that the transfer sheet 11 is fed onto the transferring belt 10 so that the transfer sheet 11 does not overlap the seam of the transferring belt 10. Thus, in such a copying machine, the processing efficiency may dramatically fall depending on the size of the transfer sheet 11, which will be detailed with referring to the following conventional copying machine.
In this conventional copying machine, the process speed (i.e., traveling speed of the transferring belt) is set to be 120 m/s, and the length of the transferring belt is 780 mm, and the remaining factors are set as shown in Table 1. In Table 1, the required return time is the minimum required time for preparing the transferring of the images, and is determined by, for example, a time in which the scanner returns to the home position immediately after the scanner has completed to read the original. The minimum paper interval is the minimum interval of the transfer sheets, and is calculated by multiplying the required return time by the process speed (i.e., the traveling speed of the transferring belt). The designated paper interval is an actual paper interval, and is determined from the whole length of the belt and the number of the transfer sheets which can be placed on the whole length of the belt such that each of the transfer sheets does not overlap the seam taking the minimum paper interval into consideration.
In the copying machine, the following is a scenario if A4 size paper having a dimension of 297 mm.times.210 mm, which is the most popular paper in many offices, is fed such that the transverse direction of the paper coincides with the traveling direction of the transferring belt. If the A4 size papers 11 are consecutively fed onto the transferring belt 10 such that the leading edge of the initial paper 11 is located within the region of 50 mm or less from the seam 10a of the transferring belt 10 so as to avoid the seam, and then the subsequent papers 11 are fed at the interval of 50 mm (designated paper interval), the forth paper 11 is fed on the same position of the transferring belt 10 on which the initial paper 11 is fed. As a result, the papers are consecutively fed on the transferring belt avoiding the seam 10a. Accordingly, the images can be transferred to three pieces of papers 11 per one cycle of the transferring belt 10 with high image quality. In other words, the images can be transferred to twenty-eight pieces of papers 11 per minute with high efficiency.
On the other hand, in a case where A4 size paper 11 is fed such that the longitudinal direction of the paper 11 coincides with the traveling direction of the transferring belt 10, the papers 11 can be consecutively fed on the transferring belt 10 avoiding the seam 10a. Accordingly, the images can be transferred to two pieces of papers 11 per one cycle of the transferring belt 10 with high image quality. In other words, the images can be transferred to eighteen pieces of papers 11 per minute with relatively high efficiency.
However, in a case where A3 size papers 11 are fed such that the longitudinal direction of the paper 11 coincides with the traveling direction of the transferring belt 10, if the initial paper 11 is fed on the transferring belt 10 immediately after the seam 10a has passed, the second paper 11 overlaps the seam 10a of the transferring belt 10 even if the second paper 11 is fed at the minimum interval. This results in unfavorable image transferring. Accordingly, it is required to feed the second paper 11 at the interval of 360 mm after the seam 10a has passed. As a result, the image can be transferred to only one piece of paper 11 per one cycle of the transferring belt 10, in other words, the image can be transferred to nine pieces of papers 11 per minute with poor processing efficiency.
As is apparent from the above, in a case where A4 size paper is fed in the transverse direction thereof or in the longitudinal direction thereof, since the difference between the minimum paper interval and the designated paper interval is relatively small, the processing efficiency is high. However, in the case where A3 size paper is fed in the longitudinal direction thereof, since the difference between the minimum paper interval and the designated paper interval becomes large, the processing efficiency becomes low. In a case where A3 size paper is fed in the longitudinal direction, the processing efficiency is only one third the processing efficiency when A4 size paper is fed in the transverse direction thereof.
Further, in a case where A3 size paper is fed in the longitudinal direction thereof, not only the processing efficiency is low, but also the lifetime of the copying machine its self becomes short because of the following reasons. Though the minimum paper interval is 86 mm, the actual interval is required to be 360 mm, which means that the length of 274 mm of the transferring belt is not utilized. During the transferring belt of the 274 mm length is traveling, the photosensitive drums, the transferring belt, and other devices are kept rotating. The rotational time per paper becomes longer, which shortens the life of the copying machine.
Even if the whole length of the transferring belt is determined so that the processing efficiency can be improved in a case where A3 size paper is fed in the longitudinal direction thereof, the processing efficiency of another size paper becomes low. Thus, the processing efficiency cannot be essentially improved.